The present invention relates to a method for preventing any stratification of the liquefied gases within a storage tank, and more particularly the present invention relates to improvements in and relating to a method and apparatus for introducing liquefied gas into a storage tank whereby when the multi-component liquefied gases comprising different kinds of liquefied gas such as liquefied natural gas (LNG) or liquefied petroleum gas (LPG) are stored in the same storage tank, the liquefied gas already existing in the storage tank and the newly introduced liquefied gas are prevented from being formed into separate strata within the storage tank due to differences in composition, density, temperature or the like.
With a multi-component liquefied gas, e.g., LNG, it is well known in the art that the gases of different origins differ in composition and density from one another and the same applies to those from the same origin but extracted on different days. When such a multi-component liquefied gas is introduced into a storage tank from a transport ship or the like, while there will be no problem if the storage tank is filled with the gases of the same composition (those of the same origin and the same extraction date), if the remaining quantity of the previously introduced gas is small so that the storage tank has a space in surplus and thus a new supply of liquefied gas of a different composition or temperature is introduced into the storage tank, the heavy and light gases do not mix together due to differences in density and these gases are formed into vertically separated strata. The formation of these strata may sometimes cause a dangerous phenomenon which is known as a roll over phenomenon.
The roll over phenomenon is one in which when multi-component liquefied gases which are different in composition and hence in density are contained in the same storage tank as mentioned previously, a stratum of higher density gas and a stratum of lower density gas will be formed as vertically separate strata; in the case of for example LNG liquefied at a very low temperature of -162.degree. C., naturally there is a heat input through the outer walls of the storage tank so that due to the nature of the boundary surface of the vertically formed two strata that it is stable and its amount of heat transfer is small, the heat is accumulated in the lower stratum of the lower density liquefied gas and the thus accumulated heat energy is rapidly dispersed in the form of evaporation of a large quantity of the liquid. As a result, there is the danger of the inner pressure exceeding the control range of the storage tank inner pressure and becoming higher than the designed limit pressure.
The density of LNG differs depending on the place of origin, that is, the density of LNG from Alaska is 424 Kg/m.sup.3, that of LNG from Brunei is 473 Kg/m.sup.3 and so on. If such LNGs having different densities are introduced into the same storage tank, a vertical stratification will be caused by the differences in density and the above-mentioned roll over phenomenon may sometimes be caused. Thus, the usual practice to prevent this phenomenon has been such that LNG of different origins are stored in separate storage tanks and thus even the amount of the stored gas is as small as about one fifth of its holding capacity thus leaving a sufficient space in surplus, any LNG having a different composition and hence a different density will never be introduced into the storage tank. In other words, if the previously introduced liquefied gas is in excess of about 1/5 of the holding capacity of the storage tank, any other LNG having a different composition is not introduced into the storage tank and the remaining space is left unused. While this prevents the occurrence of any roll over phenomenon, the present situation is such that the number of storage tanks installed at a harbor or along the coast is in excess of the required number exceeding the actual holding capacity so that not only a vast plot of ground is required but also the cost amounts to a huge sum due to the installation of refrigerating equipment for maintenance of the facilities.
Under these circumstances, as an anti-stratification measure, an attempt has heretofore been made such that when the amount of LNG contained in a storage tank is considerably less than its holding capacity, a filling nozzle is introduced into the LNG and LNG of a different composition is introduced to stir up and mix the two. The conventional filling nozzle used in this attempt has not been designed in consideration of mixing the liquefied gases having large differences in density and thus the jet of gas issued from the nozzle has failed to reach the free surface of the LNG within the storage tank thus making it impossible to stir up and mix the LNGs having large differences in density.